Author Topic: A different kind of idle problem (Read 4247 times)

lunchkrispy · « **on:** April 17, 2012, 07:21:18 PM »

Hello, finally going to make my first post here and of course it will be a long winded question. Every other little problem I've had in the last year and a half of owning this car has been easy to find a solution for on this board but this one has me stumped.

Talking about a '91 318is here. Basically, the car runs perfectly except for this: The idle will drop very low occasionally while it is still warming up. It is not enough that it will die and if I give it a little gas it will immediately go back to normal and not do it again during that drive until the next time I start the car cold (or very close to completely cold)
For example, a route I drive often: Start the car cold in my driveway and about 2 minutes from my house I will be doing 35mph and put the clutch in to down shift for a turn. If I do not shift quickly rpm will drop to around 400 and the car will almost die. I shift, get back on the gas and from there on out everything is back to normal. One time I kept the clutch in, coasted the turn and pulled over with the car in neutral just to see what would happen. RPM dropped to almost stalling speed like usual and it stayed there until I gave it a little gas and it went back to normal and held solid idle after that just fine. It has this problem at other times too but it seems like if my drive keeps it off idle during some critical point in warming up I won't notice it at all.

Any ideas what could be causing this? Seeing as the car doesn't actually stall is this even a problem or normal?

It has done this the entire time I've owned it and stayed the same through the following upgrades and maintenance:
Cleaned ICV
New ford injectors
All new intake rubber and throttle body heater delete
New o2 sensor
New ECU temp sensor
Ebay chip
New clutch

Not sure where to look next, I have this car so close to 100% other than this so any help would really be appreciated.

vonkamp · « **Reply #1 on:** April 17, 2012, 08:29:11 PM »

Sounds like a vaccum leak to me. Recheck your tubing and make sure all your intake bolts/nuts are tight...

DesktopDave · « **Reply #2 on:** April 17, 2012, 09:40:27 PM »

Did you do a stomp test? Might be a loose wire, knowing what the DME thinks could really help pinpoint the trouble. Could be a perfectly fine sensor with a dirty connector or internally abraded wire.

Just guessing, I'd suggest a flaky temp sensor. I know you've replaced the coolant temp sensor, but I'd test it regardless just to be sure. Here's the procedure according to the 3 series repair manual:

Intake air temp sensor @68degF 2200-2700 ohms; at 122degF 760-910 ohms.
Coolant temp sensor is very similar: @58degF 2300-2700 ohms; at 180degF 300-360 ohms.

Pins 4 & 5 in the AFM barrel connector are the intake air temp sensor.

Wrench · « **Reply #3 on:** April 18, 2012, 07:53:56 AM »

While you're at it check out the throttle position switch.

lunchkrispy · « **Reply #4 on:** April 18, 2012, 04:48:29 PM »

Nice! Thanks for the quick replies.

I can rule out intake leak - It has never done it once warm or while it is completely cold.

Stomp test gives me "no problems" code. I was really hoping there would be something.

I checked the old coolant temp sensor I pulled out and of course it tested just fine. I might need to do some looking in the manual and figure out which pins to check back at the ecu to rule out wiring.

AFM sounds like a likely culprit. I will check that sensor next.

I can check the TPS but I guess I'm not entirely sure what all the DME uses its signal for.

A more general question I have about these cars is at what point should the DME switch over to closed loop mode?

lunchkrispy · « **Reply #5 on:** April 18, 2012, 05:55:38 PM »

Ok got some numbers:

IAT - It's about 60 degrees out and I'm getting a resistance of 2800 ohms that dropped gradually while I pinched the sensor with my fingers to give it some heat. These values look correct.

While I had the air box open I checked the potentiometer. At fully open it starts around 500 ohms then runs smoothly up to a peak of around 850 ohms at about 20 degrees from closed then drops back down to about 500 ohms fully closed. This seems a little strange to me but the manual doesn't exactly give any solid numbers for this test.

TPS is starting at 1500 ohms closed goes smoothly up to 4000 ohms at about 10 degrees from WOT at which point it jumps straight to infinite through the rest of the range. Manual states 1000-4000 range approximate and also says it is not adjustable but it clearly is since it has slotted holes. Probably different because I am using an e36 manual. Is there a method for adjusting the TPS? It still has the yellow paint on the screws from the factory so I know it hasn't been messed with yet.

lunchkrispy · « **Reply #6 on:** April 20, 2012, 03:06:24 PM »

I've done some more reading and it sounds like my TPS having an open circuit for the last few degrees of travel means it is broken. Could someone please confirm for me that at WOT it should still show resistance up around 4k ohms?

Driving up to Seattle in a couple hours to watch supercross tomorrow so I'm not going to be messing with it until I get back.

colin86325 · « **Reply #7 on:** April 20, 2012, 05:19:53 PM »

Testing M42 TPS:

Harness connector disconnected, ignition on--in the wiring harness between Terminal 1 and ground should show about 5V

Harness connector disconnected, ignition off--Sensor Terminal 1 and 3 should show about 4k ohms

Harness connector disconnected, ignition off, throttle rotated from idle through WOT--Sensor Terminal 1 and 2 should continously vary from about 1k to 4k ohms without interruption

lunchkrispy · « **Reply #8 on:** April 20, 2012, 06:36:14 PM »

Yeah I'm talking about that third test for the potentiometer. Leaving town got delayed so I went and picked up a new TPS. The new one measures 1200 ohms with the throttle fully closed then runs smoothly up to 2900 ohms about 20 degrees before WOT. At that point it goes to an open circuit through the rest of the range. Per the method given in the manual that means it is bad. However these are almost the exact same results I got from the old TPS so I am starting to think that this is normal. Now I need to know if I should try a third one or if these are both fine?

lunchkrispy · « **Reply #9 on:** April 25, 2012, 08:49:42 PM »

Anyone want to throw me a bone here? Is this the correct resistance reading for the TPS?

anthonymax007 · « **Reply #10 on:** April 26, 2012, 10:31:03 AM »

Have you checked this?

http://www.m42club.com/forums/showthread.php?t=7757

Quote from: sports.racer;65413

Well I know what fixed mine and it will only help you if you hooked the mystery wire up like I did. I disconnected it and all is well.

Lower left corner of this graphic.

I had a problem similar to yours and this stopped it.

lunchkrispy · « **Reply #11 on:** April 26, 2012, 11:46:42 AM »

Thanks, I checked that a while ago when I replaced my blower motor. This problem never happens once the engine is warm.
This car has only had 2 owners previous to me who kept all of their receipts for service, almost entirely done by the dealer including oil changes. The kind of people who might not even know where the hood latch is if you know what I mean. I have yet to find a single thing weird or "messed with" on this car, unlike every other car I have ever owned.